I. Field of the Invention
The present invention relates to a metering valve particularly adapted for use in the fuel-injection system of an internal combustion engine.
II. Description of the Related Art
In U.S. Pat. No. 4,708,117 there is disclosed a fuel-injection system for the internal combustion engine of an automotive vehicle in which a single metering valve controls the flow of fuel in equal amounts to each of the combustion cylinders of the engine. In U.S. Pat. No. 4,708,117, a preferred form of metering valve includes a flat valve seat which forms the bottom of a fuel-containing chamber. The flat surface of the seat is formed with an annular groove and a plurality of passages or fuel nozzles lead from the bottom of the groove through the valve seat member into an air chamber where the fuel is mixed with air. A valve head, such as a poppet valve, for example, having a flat seat engaging surface is reciprocated into and out of a closed position in which the flat surface of the valve head is seated on the flat surface of the seat to block fluid communication between the fuel-containing chamber and the annular recess in the valve seat. The poppet valve is the armature of a solenoid which is energized and deenergized to cyclically open and close the metering valve in response to a pulse width modulated control signal generated by a computer in response to various monitored vehicle operating conditions.
During the time period required for one revolution of the vehicle engine, the valve is held open for a portion of this time period and closed for the remainder of the time period. Normal on-the-road operation of the vehicle finds the engine typically running in the range of 1500 to 2500 revolutions per minute (rpm); thus the time within a given operating cycle during which the valve is open is measured in milliseconds. For operational as well as emissions reasons, it is essential that leakage be minimized and, in order to meet standard leakage specifications for fuel injection valves, the mating valve surfaces--i.e. the flat mating surfaces of the valve seat and the poppet must be lapped flat and maintained true to an extremely high degree of precision.
It is a well-known, although not clearly understood, fact that when two accurately flat surfaces are engaged with each other, an unexpectedly large force is required to pull them directly apart. Where, as in the particular application described above, the time period during which a valve is to be held open is measured in milliseconds, it is obviously undesirable that it is necessary to overcome a substantial initial resistance to opening movement. Further, upon initial opening of the valve, fluid fuel will flow at a substantial velocity to create a region of reduced pressure between the surfaces by the so-called Bernoulli effect, further increasing the opening force and opening time.
While these problems would seem to indicate that a solution might be found by reducing the stroke of the poppet, as well as the area of contact between the valve poppet and seat to relatively narrow annular bands at either side of the groove, this approach is limited by the laws of fluid motion and by the fact that a sufficient area of contact must be maintained to minimize wear on the sealing surface by spreading the impact of the poppet over a reasonable surface area, particularly where this area must maintain its original accurately flat surface and surface finish.
The present invention is directed to a solution of the foregoing problems.